Answer. the multiple choice questions.

1.

38. +X "'\\ Two identical blocks slide along a ramp with negligible friction, as shown. The rst block has an initial speed '00 up the ramp while the second has the same initial speed on down the ramp. The coordinate system shown denes down the ramp to be the positive a: direction. While both blocks are sliding on the ramp, the center of mass velocity of the twonblock system is (A) constant and zero (B) constant but not zero (C) increasing in the :r direction (D) increasing in the +3 direction Disk Platform Before the Disk Drops After the Disk Drops A platform is initially rotating on smooth ice with negligible friction, as shown at left in the figure. A stationary disk is dropped directly onto the center of the platform. A short time later, the disk and platform rotate together at the same angular velocity, as shown at right in the figure. How does the angular momentum of only the platform change, if at all, after the disk drops? What is a justification for your answer? A It decreases. The top disk exerts a torque on the platform. B It decreases. The potential energy of the platform-disk-Earth system decreases when the disk drops. C It stays the same. Angular momentum is a conserved quantity. D It stays the same. The torques that the disk and platform exert on each other are equal in magnitude but in opposite directions